Method of testing electric cables



July 4, 1950 R; D. GAMBRILL METHOD OF TESTING ELECTRIC CABLES 3 Sheets-Sheet 1 Filed Dec. 24, 1946 INVENTOR R0. GAMER/LL A TTORNEY July 4, 1950 R. D. GAMBRILL METHOD OF TESTING ELECTRIC CABLES 5 Sheets-Sheet 2 Filed Dec. 24, 1946 INVENTOR RD. GAMER/LL By' W A TTORNEV y 1950 R. D. GAMBRILL 2,513,794

METHOD OF TESTING ELECTRIC CABLES Filed Dec. 24, 1946 3 Sheets-Sheet 3 lNl/E/VTOR RD. GA MBP/LL 4 7' TOQNE V /50 /aa FIG. 6

Patented July 4, 1950 UNITED STATES PATENT OFFICE 2,513,794 METHOD or TESTING ELECTRIC CABLES Richard D. Gambrill, Baltimore, Md., assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Application December 24, 1946, Serial No. 718,229

2 Claims.

This invention relates to methods of testing electric cables, and more particularly to methods of testing lightning protected, coaxial telephone cables.

A coaxial cable is a carrier type of cable now being used extensively in long distance telephone service and to some extent for transmitting both the visual images and the sound for television programs. Such coaxial cables generally consist of a plurality of individual coaxial units formed into a composite conductor core and which is covered with insulation and enclosed in a lead sheath. Long distance telephone cables frequently are buried in the earth, and, therefore, are subject to damage by lightning unless protected therefrom.

It has been found that effective lightning protection of such cables may be obtained by surrounding the lead sheathed cable with a suitable thermoplastic insulating jacket and applying therearound a tubular copper shield, which is in turn covered with a suitable protective material, such as Leno cloth tape. With this type of lightning protected cable, the current produced by a lightning strike flows along the outer copper shield, and is prevented by the thermoplastic layer from entering the lead sheath and damaging the cable sheath and conductor core enclosed thereby.

In the manufacture of this type of lightning protected cable, suitable lengths of lead sheathed coaxial cable are advanced through a machine which applies the thermoplastic insulation, the copper shield and the Leno cloth tape in successive operations, after which each length of finished cable is wound on a reel. Each reel of cable is electrically tested, the final test being one which is conducted to determine the dielectric strength of the insulation between the lead sheath and the copper shield through the application of a very high D. C. potential across the lead sheath and the copper shield.

Should there be a fault in the thermoplastic insulation, such as a conductive particle embedded therein or a dent in the shield which produces a path of reduced dielectric strength between the shield and the sheath, the insulation will break down at such a point and a high current will flow therethrough. When such a breakdown occurs it is necessary to locate the position of the fault in order that it may be repaired. The present invention relates to methods of and apparatus for locating the position of faults in the insulating jackets of lightning protected cables.

An object of the invention is to provide new and improved methods of testing cables.

A further object of the invention is to provide new and improved methods of locating the position of faults in lightning protected coaxial cables.

One app ratus embodying the invention, which may be used for practicing a method embodying the invention, comprises means for converting a low A. C. potential into a high D. C. potential,

which is applied across the copper shield and lead sheath of a lightning protected coaxial cable to break down a fault in the insulation there-between and cause a current to flow from the sheath through the fault to the copper shield. The apparatus is provided with probing means arranged to engage the copper shield and provide a shunt path for a portion of the :current flowing therein, means for indicating when the probes are engaging a current carrying portion of the shield and vice versa, and a low potential ohmmeter circuit arranged to cause a selected value of current to flow through the fault, whereby slight pressure applied against the copper tube in the vicin-' ity of the fault will decrease the resistance of the fault and cause a noticeable change in the current flowing in the circuit.

A clear understanding of the invention will be had from the following detailed description of an apparatus forming a specific embodiment thereof, when read in conjunction with the appended drawings, in which:

Fig. 1 is a circuit diagram of an apparatus constituting one embodiment of the invention;

Fig. 2 is a front, elevational view of the housing which encloses the apparatus shown in Fig. 1;

Fig. 3 is an enlarged, vertical, sectional view of a portion of the apparatus shown in Fig. 1;

Fig. 4 is an enlarged, partially sectional view of another portion of the apparatus shown in Fig. 1;

Fig. 5 is a vertical, end view of the apparatus shown in Fig. 4;

Fig. 6 is a horizontal, sectional view taken along line 68 of Fig. 5, and

Fig. 7 is a vertical sectional view taken along line 1-1 of Fig. 6.

Referring now to Fig. 1 of the drawings, it will be noted that there is shown a portion of a lightning protected coaxial cable indicated generally at l0, which for the purpose of clarity will be assumed to be wound on a conventional cable reel in which the left hand end of the cable 10, designated A, will be considered to be the outer end of the cable, while the right hand end, designated B, will be assumed to be the inner end reduced dielectric strength between the lead sheath I 2 and the copper shield l4. The faults most likely to occur in the insulating jacket l3 are metallic particles embedded in the insulation or a dent in the copper shield which reduces the amount of insulating material between the copper shield and the lead sheath. In either case, when the cable is subjected to a high D. C. potential to determine the condition of the insulating Jacket I3, the fault will cause the jacket to break down at that point. While these faults may take the forms referred to hereinabove, or may be of another nature, such a fault between the copper. shield and the lead sheath is indicated generally at |3 as a low resistance path therebetween. If a breakdown should occur at the .fault I3, it is necessary to locate the position of the fault so that the cable may be repaired at the defective point, in which case testing apparatus and method embodying the invention are employed to locate the position of the fault with respect to the outer end A of the cable l3 in a manner to be hereinalter described.

A testing apparatus embodying the invention comprises a compact-portable testing apparatus |3 (Fig. 2) having an instrument and control panel -|3 on which all the indicating meters and control levers of the apparatus are assembled. The testing apparatus comprises a single throw, double pole switch 23 (Fig. l) which serves to connect stationary contacts 2| and 22 of a magnetic relay 23 and a primary winding of a stepdown transformer 23 to a suitable source of A. C. potential (not shown). Normally open arms 4 23 and 21 of the relay 23 are connected to a primary winding 33 of a stepup plate transformer 3| which contact arms, when in engagement with the stationary contacts 2| and 22, serve to connect the primary winding 33 directly across the A. C. power supply. The stepdown transformer 25 has its secondary winding 32 connected to supply a low A. C. potential across control busses 33 and 34. A conventional motor driven timer 33,

such as a Cramer, 'I'D-2, l23-S timer, is .con-

nected directly acrossthe control busses 33 and 34 serves to control the continuity of an operating coil 33 of the relay 23. The transformer 3| is provided with a tapped secondary winding 33,

A tap a (Fig. 1) of the secondary winding 36. is connected to a plate 44 of a mercury vapor rectifier 43. A ten 43 provided on the secondary winding 33 is connected to a plate 41 provided on another mercury vapor rectifier 43, which is identical in size and rating with the rectifier 45. Filaments 33 and 3| of the rectiilers 43 and 43, re-

4 spectively. are connected in parallel and directly across a secondary winding 32 of a conventional filament transformer 53 having its primary winding 34 connected across the control busses 33 and '34. The secondary winding 32 is provided with a tap 33 to which is connected one end of a current limiting resistor 33 whose other end is connected to another current limiting resistor 31 which in turn is connected to a stationary contact 33 provided on a magnetic relay 33. A normally open contact arm 33 of the relay 33 is connected directly to a movable arm 3| forming part of a four-position, four-deck, manually operated selector switch 32.

The switch 32 (Fig. 1) is shown on the drawing in its normal operating position in which the arm 3| engages a first position contact 33 of its respective deck. The contact 33 is connected by lead 34 directly to a spring clamp 33 internally disposed ln the hood 42, and arranged to engage a prepared end of the lead sheath l2 at the outer end A of the cable M. The right hand terminal of the ammeter 33 is connected directly to another arm 33 of the selector switch 32 by means of a conductor 31. Each of the four-contact positions engaged by the arm 33 of the switch 32 serves to connect one side of the ammeter to ground through suitable resistors 33--63, which ground is the same ground as that indicated by the numeral 33. I

The combination of the stepup transformer 3| (Fig. 1) and the rectiflers 43 and 43 connected to the secondary winding thereof serve to step up the A. C. potential supplied to the transformer and convert the secondary potential to a relatively high D. C. potential" of the order of 4500 volts. This high D. C. potential is applied across a .circuit, the grounded side of which starts at the tap 31 of the transformer 3|, and continues through the ammeter 33, the lead '43 and the clamp 4| of the hood 42 which engages the copper shield l4. The high potential side of the circuit starts at the tap 33 on the secondary winding 32 of the transformer 33 and continues through the resistors 33 and 31 to the contact 33 of the relay 33 and from the arm 33 thereof through the arm 3| and contact 33 of the switch 32 to the clamp 33 which engages the lead sheath l2 of the cable |3. A pair of condensers 33-33 is connected directly across. this high D. C. potential circuit ahead of the ammeter 33 and the resistor 51.

An operating coil 13 (Fig. 1) of the relay 33 has one side thereof connected directly to the control -bus 34 and the other side thereof connected to one side of a manually operable switch 1| mount ed in a probe holder 12. The other side of the switch 1| is connected directly to one side of another manually operable switch 13 mounted in a second probe holder 14, which switch .has the other side thereof connected directly to the control bus 33. Hardened steel probes 13 and 13 are secured in the ends of the probe holders 12 and 14, respectively, in a manner hereinafter to be described. Since the probe holders 12 and 14 are identicalin construction, only the probe holder 12, shown in Fig. 3 of the drawings, will be described in detail. The probe holder 12 comprises a metallic tubular housing 11 in which the switch 1| is mounted so that its operating lever protrudes beyond the outer periphery of the housing. An annular head 13, made of suitable electrical insulating material, is secured to one end of the housing 11 and has a metallic post 13 secured in a central aperture provided therein. The post 13 is provided with an integral flange by means of which the post is secured to the head 13 and a central aperture in which the probe I is adjustably positioned. The head I8 serves to insulate the metallic probe I5 from the metallic housing I1, whereas the post I8 provides a contact surface within the housing II which may be electrically connected to a probing circuit 88 to be hereinafter described. The opposite end of the housing TI has a metallic head secured thereto on which is mounted a conventional cable grip.

The probes I8 and 18 (Fig, 1) are-provided with very sharp points so that they may pierce the covering of Leno cloth tape I8 and engage the copper shield I4. The housings ll-11 or the probe holders 12 and 14 are connected together and to ground in order to protect an operator when the probes I5 anad 18 mounted therein are in engagement with the current carrying portion of the shield I4. It will be noted that the coil I8 of the relay 8| is connected directly across the busses 33 and 34 by the manually operable switches II and 18 in which case they control the continuity of the circuit supplying potential to the coil I8. An indicating lamp 88 is connected directly across the terminals of the operating coil 18 of the relay 88 and serves to indicate when the coil 18 is energized by the closure of the switches II and 13.

In the probing circuit 88 (Fig. 1) the probe I5 of the probe holder I2 is connected to one of the input terminals of a conventional, IL C. operated, three-stage, D. C. amplifier indicated generally at 88 by a conductor 8|. The probe '76 is similarly connected to the other input terminal of the D. C. amplifier 88 through a pair of contacts 82 and 83 arranged to be periodically bridged by a cam 84 driven by a motor 85 forming part of a timing apparatus 88. The D. C. amplifier 88 may be of any conventional design; but the type used with this apparatus is the type 715-A, Direct Current Amplifier, manufactured by the General Radio Company. The A. C. operating potential of the D. C. amplifier 88 is obtained from the control busses 83 and 34 and passes through a conventional voltage stabilizer 81 in order to maintain a constant A. C. operating voltage to the amplifier 88.

A microammeter 88 (Fig. 1) (having a range 0 200 microamperes) is connected in the output circuit of the amplifier 88 in series with a sensitive operating coil 8| of a single pole, double throw relay 82. The A. C. operating circuit of the D. C. amplifier 88 is adjusted so that the indicating needle of the microammeter 88 assumes the mid-scale position when the potential across the input terminals thereof is zero, that is, when the potential across the probing circuit 88 is zero. The D. C. amplifier is adjusted in this manner because the microammeter 88 is used to indicate merely the existence of a potential difference across its input terminal and not to indicate the actual value of the potential.

when a. D. C. amplifier of the type described is used in this manner, it is generally referred to as 9. "Vacuum Tube Microammeter. The method of obtaining the zero position of the indicating needle of the microammeter 88 and its operation are fully disclosed and described at pages 129 to 132, inclusive, of the book, "Vacuum Tube Voltmeters, by John F. Rider, published by John F. Rider Publisher, Incorporated, New York, 1945.

The microammeter 88 (Fi 1) is provided in the output circuit of the amplifier 88 in order to have a very sensitive indicating means connected across the probes l8 and '18. This sensitive indicating means, consisting of the amplifier 88 and the microammeter 88, is needed to give a substantial indication of the very small potential drop existing across the portion of the copper shield engaged by the probes I5 and 18. In other words, a very slight change in the potential applied across the input terminals of the amplifier will eflect a substantially greater defiection of the indicating needle of the microammeter 88. For this reason, the current responsive relay 82 is provided for protecting the microammeter from excessive output currents which may occur during the warm-up period of the amplifier orfrom the probing circuit 88 during a search for the approximate position of the fault I8 in the insulating jacket.

Since the operating coil 8| of the relay 82 is connected in series with the microammeter 88, it is also subjected to the output current of the amplifier. When an overcurrent condition occurs, the coil 8| actuates an arm 83 so that it engages a contact 84 Or a contact depending upon the polarity of the input potential. The relay 82 is provided with an electrically operated latch 88 which serves to latch the arm 83 in its respective actuated position. The latch 88 is operated by a reset coil 81 having one side thereof connected directly to the bus 33 and the other side connected to one side of a reset button 88, the other side of which is connected to the bus 34.

The arm 83 is connected directly to the bus 33, whereas the contact 84 is connected to one side of a coil I88 of a relay I8I while the contact 85 is connected to one side of an operating coil I82 of a relay I83. The coils I88 and I82 have their opposite sides connected together and to the bus 34. A contact I84, provided on the relay I8I, and a contact I85 provided on the relay I83 are connected in parallel with each other, one side of the parallel circuit being connected directly to one of the output terminals of the amplifier 88 while the opposite side thereof is connected to the movable terminal of a variable resistor I88 which is in turn connected to the other output terminal of the amplifier. In other words, either of the relays IM and I83, when energized by operation of the arm 83 will connect the resistor I88 directly across the output terminals of the amplifier 88, thereby shunting out the microammeter 88 and protecting it from damage by excessive output currents.

The relay I8I is provided with a second normally open contact I85 which serves to connect an indicating lamp I81 across the control busses 33 and 84 when the relay is energized. The relay I83 also is provided with a second normally open contact I88 which serves to connect an indicating lamp I88 across the control busses 33 and 34 when the relay is energized. The indicating lamps I81 and I89 serve to indicate not only that the current relay 82 has been actuated by an excessive output current of the amplifier 88, but also the polarity of the output current so that the proper A. C. controls of the amplifier may be adjusted to. correct the overcurrent condition.

The motor 85 of the timer 88 is connected directly across the control busses 33 and 34 through a contact H0 and an arm III of the selector switch 62. An indicating lamp 2 is also connected across the control busses 33 and 34 by means of the contact H8 and the arm III of the switch 82 serves to indicate when the probing circuit 88 is being used tolocate the approximate position of the fault I6. The timing'motor 22 also drives a cam II2, which'serves to bridge a pair of contacts II4-I I4 and thereby control the continuity of an indicating lamp I I which is connected directly to the control busses 22 and 24 through the timer contacts I I4,I I4. Another indicating lamp II6 has one side thereof connected to a second position contact II1 which the cam I I2 provides a simultaneous intermittent energization of the indicating lamp iii. In this manner the lighting of the lamp II indicates the simultaneous closure of the probing circuit. Each time the cam 24 bridges the contacts 22 and 22 the potential difference existing between the probes and 16 engaging a given length of the copper shield I4 is applied across the input terminals of the amplifier 20, which potential difl'erence will causethe indicating needle of the microammeter 90 to deflect either to the right or to the left of its preset mid-scale position, depending upon the polarity of the potential input to the amplifier. The intermittent operation of the indicating lamp 5 is provided so that the operator of the testing apparatus should observe the movements of the microammeter only when the indicating lamp II! is lighted.

It is believed to be obvious that as long as the probes 15 and 16 are positionedon a given length of the shield I4 between the outer end A of the cable and the fault I6, there will be sufllcient potential difierence therebetween to cause a suitable deflection of the microammeter 90. However, when the probes 15 and 16 are moved from the outer end A toward the inner end 3, they may assume a position beyond the fault I6 in which case no potential diflerence would exist between the probes and, consequently, no deflection would occur in the microammeter.

It is believed to be apparent that the probes 15 and 16 must be spaced a suitable distance apart so as to include a substantial portion of the shield I4 and the current flowing therein in order to obtain a potential drop therebetween sufllcient to render a suitable deflection on the microammeter. Due to this required spacing of the probes 15 and 16, it is obvious that as the probes are spaced at various points along the shield I4, they may ultimately be spaced so as to straddle the fault I6, in which case the potential drop between the probes would be substantially reduced and a correspondingly slighter deflection would occur in the microammeter.

It has been found that in order to obtain a potential difierence across the probing circuit 89 which will give a suitable deflection on the microammeter 20, the probes 16 and 16 must be spaced on the cable about six feet apart. With this spacing of the probes 15 and 16, it is impossible to accurately determine the position of the fault IS, the nearest location of the fault bein at some point within the six feet spacing of the its secondary-winding I22=is connected to plates I22 and I24 of a mercury vapor rectifier I25 through resistors I26 and I21, respectively. The other side of the winding I22 is connected directly to one side of a resistor I22. A secondary winding I20 of the filament transformer 52 has the ends thereof connected to filaments I 2I-I2I of the rectifier I25. A tap I22 provided on the secondary winding I20 is connected to the opposite end of the resistor I22, which end is also connected directly toa spring clamp I22 disposed within a hood I24 which is similar to the hood '42 and which is positioned over the inner end B of the cable I0 so that the clamp I22 engages a prepared end of thelead sheath-I2. The hood I24 is also provided with an externally operated spring biased clamp I25 like the clamp 4I provided on the hood 42, which clamp grips the outer periphery of the cable I0 and thereby serves to hold the hood on the inner end B of the cable. The described connections of the rectifier I25 serve to supply half-wave rectification of the secondary potential of the transformer. I22 directly across the resistor I22.

The resistor I22 is provided with a movable tap I26 which is connected directly to an arm I21 of the selector switch 62. When the switch 62 is in such a position that the arm I21 engages its respective first position contact I22, the ohmmeter circuit is open circuited. When the arm I21 engages either its second, third or fourth position contacts I42, I 4| or I42, respectively, the ohmmeter circuit is then connected to the ground 22 by a conductor I43, in each case, through an associated variable resistor I44, I45

orl46.' The arm I21 serves to selectively con-' nect the resistors I44, I45 or I46 in the ohmmeter circuit in series with the fault I6, and as the arm I21 is moved from its second position" contact I40 to its fourth position contact I42, the resistors I44, I45 and I46 are of such value that a correspondingly higher resistance is connected in the ohmmeter circuit, and consequently, in series with the fault I6.

The ohmmeter circuit applies a pulsating D. C.

potential across the lead sheath I2 at the innerend B and the copper shield I4 at the outer end A of the cable I2 causing current to flow from" the tap I22 of the winding I20 to the lead sheath I2 at the inner end B of the cable, along the lead sheath I2 to the fault I. in the insulating jacket I2, through the fault I6 to the copper shield I4, along the copper shield to the outer end A of the cable. The circuit continues from the shield at the outer end A of the cable through the ammeter 22 and the conductor I42 to the common connection of the resistors I44, I45 and I46, through the particular resistor which is connected to the contact engaged by the arm I21,

of the resistor I22 to one side of the secondary winding I22. When the arm III is moved to en-' gage the second position contact I I1 or the third or fourth position contacts connected thereto,

the lamp Iii is placed across the control busses 22 and 24, thereby energizing the lamp lit to indicate that the ohmmeter circuit is now conprobes, when said probes are positioned so as to straddle the fault.

To obtain an accurate location of the fault I6,

a low potential ohmmeter circuit is provided, which includes a stepdown transformer I20 having its primary winding |2I connected directly nected to the cable I0 as described above.

Since the contact hoods 42 and I24 are identical in construction, only the contact hood 42.

across the control busses 22 and 24. One side of 22 sulating material having an annular head I5I,

made of suitable insulating material, secured thereon. A support I 52, made of suitable insulating material, is secured to the annular head II and has the spring biased clamp 65 securely mounted thereto. The support I52 is positioned on the annular head I5I so that the jaws of the clamp 65 straddle the central axis of the contact hood 42 and, consequently, the tubular body I50. A bearing I53, made of suitable insulating material. and having a bore I54 therethrough is mounted against the inner wall of the body I50 and against the annular head I5I in such a manner that the bore I54 is aligned with a bore I55 provided in the tubular body I50, the central axis of which is diametrically perpendicular to the movable arm I 56 of the clamp 65 mounted on the support I52. I

A push rod I51 (Fig. 4) is slidably positioned in the bores I54 and I55, having one end thereof projecting inwardly beyond the bearing I53,so as to engage the arm I56 and the other end thereof protruding outwardly beyond the periphery of the body I50 in order that the push rod may be urged inwardly to operate the arm I56 of the clamp 65 when so desired. A metallic support I58 is secured on the outer surface of the annular head I5I and diametrically aligned with the push rod I51. The support I58 is provided with a tapped bore I60 adjacent to the periphery of the body I50 for receiving the threaded portion of the conventional cable grip I6I and has the opposite end I 62 thereof turned inwardly toward the opposite end of the body I50 in order to form a guard over the end of the push rod I51. The turned over end I62 of the support I51 serves to prevent accidental operation of the push rod I51 and, consequently, the spring clip 65.

A pair of diametrically opposed cut-outs I63-I63 (Fig. 4) provided in the body I50 adjacent to the annular head I5I are covered by a pair of arcuate-shaped members I64I64, made of suitable transparent plastic material, thereby providing a pair of windows in the tubular body I50 at the portion of the body which encloses the clamp 65. In other words, the arcuateshaped members I64-I64 form a pair of windows by means of which the operator of the hood 42 can readily determine when the clamp 65 properly engage the prepared end of the lead sheath I2 when the hood is positioned over the outer end A of the cable I0.

The clamp 4| (Figs. 4 and 5) which engages the copper shield I4 of the cable I0 is mounted on the opposite end of the body I50 in a manner hereinafter to be described. The clamp 4| comprises a pair of arcuate-shaped jaws I65 and I66 having saw teeth formed on the inner edges thereof. 'Each of the jaws I65 and I66 has one end thereof pivotally mounted on spaced studs I61 and I68, respectively, which studs are threadedly mounted iii a support I10 rigidly secured in an upright position on a ring IN. The ring "I is positioned over the end 01 the body I50 so that the end of the ring "I and the end of the body I50 are flush with each other. A rectangular plate I12. is positionedon the studs I61 and I68 and spaced from the. support I10 by a tubular spacer I 13 positioned over .the stud I61 and against an outer surface I14 of the support I10 and a tubular spacer I 15 positioned over the stud I68 and against an undercut surface I18 of the support I10.

The end of the jaw I65 which is pivotally mounted on the stud I61 is positioned on the the support I10 (Fig. 6) The length of the spacer I13 is such that a free-sliding movement of the pivoted end of the jaw I65 between the plate I12 and the support I10 is obtained. The end of the ,jaw I66 which is pivotally mounted on the stud I68 is positioned between the pivotally mounted end of the jaw I65 and the undercut surface I16 of the support I10. The length of the bushing I15 positioned on. the stud I68 is such that a free-sliding movement of the pivoted end of the jaw I66 between the jaw I65 and the support I10 is obtained.

A hardened steel pin I11 (Figs. 6 and 7) is provided with a cylindrical body portion I18, a portion of which body portion is rigidly secured in a bore I provided in the pivotally mounted end of the jaw I66, and a portion of which extends beyond the jaw I66 and slidably engages an elongated aperture I8I provided in the end of the jaw I65 adjacent to its pivot point. The opposite end of the pin I11 is rrovided with a ball-shaped end I82 which protrudes through an elongated slot I83 (Figs. 5 and 7) provided in the support I10 and a similar slot I84 provided in a leg I85 of a U-shaped handle I 86 which is secured to the support I10 and the ring I1I. The end I82 engages an oval-shaped socket I81 provided in a crank I88 forming part of a manually operable lever I89 (Figs. 4 and 7).

The lever I89 is pivoally mounted between opposite walls of the leg I85 of the U-shaped handle I86, having a channel-shaped cross section, by means of a stud I90. A grip I9I is secured on the outer surface of the U-shaped handle I86 in order to provide better leverage on the lever I88. A torsion spring I92 is centrally positioned around a bushing I93 positioned on the stud I61 and under a washer I94 secured under the head of the stud I61. One end of the spring I92 is anchored around a bushing I95 positioned between the head of the stud I68 and the plate I12 and the other end of the spring I92 engages a pin I96 rigidly secured in the jaw I65. The bushing I93 permits free movement of the coils of the spring I92 between the washer I94 and the plate I12 when the jaws I65 and I66 are actuated b the lever.

The spring I92 tends to maintain the jaws I64 and I66 in a semi-closed position against the periphery of the shield I4 as illustrated in Fig. 5. However, it is believed to be apparent that when the lever I89 is urged toward the under side of the handle I86, the crank I88, being in engagement with the ball-shaped end I82 of the pin I11, urges the pivoted ends of the jaws I65 and I66 toward the central axis of the contact hood 42. In other words, by actuating the lever I89, the free end of the jaws I65 and I66 may be made to assume their respective broken line positions shown III Fig. 5.

The outward movement. of the free ends of the jaws I65 and I66 is against the action of the torsion spring I92, whereupon when the lever I86 is released, the spring I92 urges the free ends of the jaws inwardly whereupon their respective sawtoothed edges engage the periphery of the copper shield I4 of the cable I0 (Fig. 6) when the contact hood 42 is positioned on the outer end A of the cable as shown in Fig. 5'. A. flexible lead I91 is secured in the cable grip HI and contains the conductor 64 which is secured to the clamp 65 and the conductor 40 which is secured in a suitable manner to a ring I98 mounted on the inside of the tubular body I50. The inner ring I98 and the outer ring I1I are secured to the end of the Operation The above-described apparatus operates in the following manner:

The portable testing apparatus ll described hereinabove is positioned adjacent to a reel containing lightning protected cable, such as the cable "I shown in Fig. 1 of the drawings, and the contact hoods 42 and I34 are positioned on the outer end A and the inner end B of the cable. respectively, thereb connecting the lead sheath [2 and the copper shield l4 thereof to the testing apparatus. Let it also be assumed that the fault it in the insulating jacket I: was previously ascertained by subjecting the cable It to a high voltage breakdown test, in which approximately 15,000 volts D. C. was applied across the copper shield l4 and the lead sheath 12 so as to burn in or char any faults that may be present in the insulating jacket i3.

The testing apparatus is then connected to a suitable low potential A. 0. supply so that when the switch -20 is closed, the transformer 25 is energized thereby supplying a low A. C. potential across the control busses I3 and 34. The timer 35 also is energized upon the closure of the switch 20 and begins its timing cycle, which, when completed, energizes the operating coil 36 of the relay 23 causing the contact arms 2 and 21 to engage the stationary contacts I! and 22 thereby energizing the primary winding 34 of the stepup transformer ii. The transformer I3 is energized simultaneously with the timer l5. and while the timer is completing its timing cycle, the filaments 50 and ii of the rectifiers 45 and 44, respectively. being connected to the secondary winding of the transformer it heat up the rectiflers and 48 and place them in operating condition. When the timer completes its ttming cycle, the rectifiers 45 and 4a are sufliciently heated up and are in condition to have the high A. C. potential produced by the secondary winding 36 applied across the plates 44 and 41 thereof.

The rectifiers 48 and 44 serve to provide full wave rectification of the secondary potential of the transformer gland supply a high D. C. potential (approximately 4500 volts D. C.) across the D. C. circuit beginning at the tap 31 on the secondary winding 36 and including the ammeter 3!, the copper shield 14 of the lightning protected cable II, the fault I, the lead sheath I 2, the arm ii of the selective switch 02, the contact arm 50 of the relay II, the current limiting resistors 68 and I1 and ending at the tap it provided on the secondary winding '2 of the filament transformer 53. This high potential circuit connected to the lead sheath l2 and the copper shield l4 is open circuited until the switches Ii and I3 provided on the probe holders 12 and 14, respectively, are closed, in which case the coil II is connected directly across the control busses It and 34, thereby energizing the coil II and causing the contact arm 60 to engage the contact I4.

When the switches 1i and It are so operated, the high D. 0. potential supplied by the transformer II and the rectifiers 4i and 4! is applied '12 directly across the copper shield l4 and the lead sheath I! of the lightning protected cable II and consequently across the fault it which exists in the insulating jacket ll positioned between the lead sheath and the copper shield. The application of the 4500 volts D. C. potential across the lead sheath and the copper shield will break down the previously burned-in fault it in the insulat n jacket II and cause a current to flow from the tap II to the lead sheath I: at the outer end of the cable ll, along the lead sheath ii to the fault I, through the fault to the copper shield l4, back along the copper shield to the outer end A thereof. and thence through the ammeter 39 to the tap 31 on the transformer ll. Since the resistance of the fault It depends upon the extent to which it was burned-in by the high voltage test, the desired current flow through the fault is obtained by connecting the rectifiers 45 and 48 to the taps provided on the transformer II which will produce this current. When the desired current fiow has been established through the fault I. the probe holder 12 is positioned as close to the outer end A of the cable It as is permitted by the contact fixture 42 secured thereon so that the probe Ii pierces the "Leno cloth tape covering l5 and contacts the copper shield l4. The probe holder 14 is positioned on the cable II in a like manner approximately six feet from the probe ll in the direction of the inner end B of the cable I. so that its probe 16 contacts the copper shield.

When the probes II and I0 engage the copper shield i4 in this manner, the probes and the probing circuit 04 associated therewith provide a shunt path for a portion of the current flowing along the copper shield i4 from the fault it back to the outer end A of the cable. A potential difference exists across probes l5 and 16, which is directed to the input terminals of the D. C. amplifier 00 through the contacts 42 and provided on the timer 46. The timer It is energized simultaneously with the timer 3! and the filament transformer 53 and continues to rotate the cams 84 and III as long as the arm III of the switch 62 engages the first position contact Ill.

Each time the cam 84 bridges the contacts 02 and 83, the potential difference across the probes i applied to the input terminals of the amplifier, which potential disturbs the stable presetoperating condition of the amplifier and effects a substantial change in the output current thereof,

, causing the microammeter I. connected in the output circuit of the amplifier to be deflected to the right or to the left of its preset mid-scale position depending upon the polarity of the potential difference. It should be noted that when the cam 84 is bridging the contacts 42 and II,

the cam ll! simultaneously bridges the contacts ll4-ll4 of the-timer I6 and causes the lamp Hi to be lighted, thereby indicating that the potential diflerence across the probes is applied to the input terminals of the amplifier only when the indicating lamp III is lighted. When the probes contact the current carrying section of the shield i4 in this manner, each time the indi-' cating lamp I I5 is lighted, there is a simultaneous deflection of the indicating needle of the microammeter 90, which is noted by the operator of the test set.

After several deflections have been registered by the microammeter in synchronism with the lighting of the lamp I ii, the probe holders l2 and 14 are moved from the outer end A of the cable along the outer layer of the cable on the reel, and at each test position of the probes II and It there is maintained a length of copper shield therebetween of approximately six feet. In other words, as the probes are moved along the outer layer of the cable I to various test positions along the shield, the spacing between the probes is maintained fairly constant throughout the probing test. If, after probing the entire outer layer of the cable III, it is determined that the fault I6 is not in the outer layer of the cable.

then the outer layer must be unwound from the reel so that the probes I and I6 may engage the copper shield of the next layer of the cable. In any event, regardless of the position of the fault I6 in the cable II). the probes are moved from the outer end A toward the inner end B, until a posilion is reached where, with the probes engaging the copper shield I4, no indication of a potential difference between the probes is indicated on the microammeter 90.

When this point has been reached, it is believed to be obvious that the probes I5 and I6 are now positioned on the non-current carrying section of the copper shield I4, that is, the probes are positioned on the shield I4 between the fault I6 and the inner end B of the cable. The probes l5 and I6 are then moved together in small increments back toward the outer end A of the cable until a point on the cable is reached where each time the indicating lamp 95 is lighted, a slight movement of the indicating needle of the microammeter 90 is discernible. This slight movement of the indicating needle" indicates that the probes l5 and I6 now straddle the fault I6 as indicated by the broken line positions of the probes in Fig. 1 of the drawings. The small movement of the microammeter needle is due to the fact that, while the length of the copper shield between the probes is still approximately six feet, the probe is in all probability only three or four feet away from the fault I6. Hence, instead of measuring the potential drop across six feet of the copper shield I4, the probes are now measuring the potential drop across approximately three or four feet of the copper shield. Having established the fault to be somewhere between the final positions of the probes I5 and 16, the cable is marked at these points and the probes are disengaged from the copper shield and placed in suitable holders (not shown) provided on the testing apparatus I8.

It is desirable to determine the exact position of the fault in order that the cable III may be repaired at that point, since it is highly undesirable to break open the entire six feet established by the probes to determine the exact position of the fault. In order to accomplish this, the switch 62 is actuated so that the arms II and 66, III and I3! engage their respective second position contacts. The arms III and 68 disconnect the timer 86 from the control busses 32 and 34 and open the high potential D. C. circuit connected to and supplied by the rectifiers 45 and 48, respectively. At the same time, the arm I31 closes the ohmmeter circuit supplied with a low D. C. potential by the rectifier I and the transformer I20, which potential is applied across the inner end of the lead sheath I2 and the outer end of the copper shield I4. When the arm II'I engages its second position contact I40, it connects the resistor I44 in the low potential circuit in series with the fault Ii.

If the resistance of the fault I is such that too great a current flow in the ohmmeter circuit is indicated on the ammeter 39, the switch 2 is actuated so that the arm III engages its third position contact I, thereby inserting the higher resistance I45 in serie with the fault. Likewise, if the current flowing through the fault is still too high, the switch 62 is actuated so that the arm I31 engages its fourth position contact I42,

' which inserts the higher resistance I46 in series with the fault. In other words, the switch 62 is actuated until a suitable reading is obtained on the low range scale of the ammeter 39. The low potential circuit has been referred to hereinabove as the ohmmeter circuit because definite amounts of resistance are placed in series with the fault I6 so that the reading of the ammeter is actually a measurement of resistance of the fault.

When the proper current flow has been established in the ohmmeter circuitfan operator begins to apply manual pressure against the leno cloth covering I5 and the copper shield I4 at a point close to the final position of the probe I5 and proceed to apply this pressure at various points along the cable III toward the final position of the probe 16. As the pressure is applied at successive points between the final position of the probes l5 and I6, an intermediate point between the probes is reached, where, when pressure is applied thereat, a substantial change in the value of current flowing in the ohmmeter circuit is indicated by the ammeter 4|. This means that the pressure is being applied adjacent to, or substantially over the fault I6, thereby compressing the insulating jacket I3 at or near the fault and bringing the shield I4 closer to the lead sheath I2. This pressure against the shield at or near the fault causes the resistance of the fault to be substantially decreased, whereupon an increased current flow occurs in the ohmmeter circult.

The application of manual pressure against the cloth covering I5 and the shield I4 at various points around the fault it will finally lead to a point where, when pressure is applied thereat, the greatest change of current flow in the ohmmeter circuit will be indicated. Obviously, the point at which the application of pressure renders the greatest change in the flow of current in the ohmmeter circuit is substantially the exact position of the fault is in the insulating jacket I3. This point is marked on the cable by the operator, after which the switch 20 is opened to disconnect the testing apparatus from the supply potential and the switch 62 is returned to its normal operating position. The testing apparatus is then moved in position to conduct a similar test on another reel containing lightning protected coaxial cable, while the cable previously tested may be repaired at the point determined by the above-described test.

The contact hoods 42 and I34 described hereinabove are designed to connect the lead sheath I2 and the copper shield I4 of the lightning protected coaxial cable III to the testing apparatus I8. Referring to Figs. 1 and 4 it will be noted that the ends A and B of the coaxial cable I0 are prepared in a special manner inherent in the manufacture of such cable. This special preparation of the ends A and B is required in order that the cable may be filled with an inert gas such as hydrogen. The special preparation of the ends of the cable consists of a lead cap 200 which is burned onto the lead sheath I2 and a threaded fitting 2M which is centrally secured on the cap 200. The gas supply is connected to the fitting 2M and after the cable is filled, a plug is threaded into the fitting 2 to keep the gas in the cable. Therefore, to connect the lead aria";

15 sheath I! to the testing'apparatus, the spring.

clamps I and in were provided in the contact hoods to engage the fitting 201. Suiiicient clearonce is maintained between the cap 209 and the conductor core so that the testing current flows only in the lead sheath. It is believed to be apparent that a suitable clamp could be disposed within the contact hood for engaging the periphery of the lead sheath I! where the ends of the.

a lead sheath and a metallic shield of lightning protected cable, which comprises impressing across the lead sheath and metallic shield of such a cable a potential having an intensity suillcient to break down such a fault and cause a continuous current flow along the lead sheath to the fault in the insulation, through the fault to the metallic shield and along the metallic shield to the same end of the cable, forming a shunt path to measure the potential drop across a length of the shield at various points along the current carrying section of said shield until a portion of the shield is reached at which a substantial decrease occurs in the value of the potential drop across the shunt path, said decrease in the potential drop indicates that the fault lies within the respective portion of the shield included in the shunt path, impressing a second potential across the lead sheath and shieldhaving an intensity sumcient to cause a substantially lower current to flow along the lead sheath from one end of the cable through the fault to the shield and along the shield to the opposite end 4' of the cable, and applying pressure against the shield along that portion thereof in which the decrease in the potential drop occurred until the point where a maximum increase of current flow through the fault occurs. said maximum increase in the current indicating that the fault is directly beneath that point because the resistance of the fault is substantially reduced by the pressure applied to the shield.

2. The method of locating a previously de- '16 tected fault in the insulation positioned between a lead sheath and a' metallic shield of lightning protected cable, which comprises causing a continuous current to flow along the lead sheath from one end of such a cable through a fault in the insulation to the copper shield and along the copper shield to the same end of the cable, forming an external shunt path circuit adapted to measure the potential drop across .a selected portion of the current carrying section of the copper shield. amplifying the potential drop across the shunt path circuit, forming said shunt path circuit at various points along the copper shield until a portion of the shield is reached at which -a substantial decrease in the potential drop thereacross occurs indicating that the fault in the insulation lies within that portion of the c l causing a substantially lower value of current to flow along the lead sheath from one end ofthe cable through the fault to the shield and along the shield to the opposite end of the cable,

measuring the current flowing through the fault,

REFERENCES crrEn The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 754,402 Wiseman Mar. 8, 1904 1,407,693 Jones Feb. 28, 1922 1,754,419 Henneberger Feb. 4, 1930 1,919,802 Pfeiifer July 25, 1933 2,020,402 Edwards et al. Nov. 12, 1935 2,120,391 Butterfleld June 14', 1938 2,123,910 Peters July 19, 1938 2,125,630 .Gooding Aug. 2, 1938 2,199,106 Jones Apr. 30, 1940 2,199,846 .Borden May 7, 1940 2,420,055 Sanders May 6, 1947 2,451,953 Ingram Oct. 19, 1948 x FOREIGN PATENTS Number Y Country Date 306,679 Germany July 9, 1918 576,708 Great Britain Apr. 16, 1946 691,408 France Oct. 21, 1930 

